I don’t know any more inspiring missionary promise than this word from Jesus.
This gospel of the kingdom will be proclaimed throughout the whole world as a testimony to all nations, and then the end will come. (Matthew 24:14)
Not: This gospel should be preached. Not: This gospel might be preached.
But: This gospel will be preached.
This is not a great commission, nor a great commandment. It is a great certainty, a great confidence.
Who can dare talk like that? How does he know it will? How can he be sure the church will not fail in its missionary task?
Answer: The grace of missionary service is as irresistible as the grace of regeneration. Christ can promise universal proclamation because he is sovereign. He knows the future success of missions because he makes the future. All the nations will hear!
A “nation” is not a modern “country.” When the Old Testament spoke of nations, it referred to groups like Jebusites and Perizites and Hivites and Amorites and Moabites and Canaanites and Philistines. “Nations” are ethnic groups with their own peculiar culture. Psalm 117:1: “Praise the Lord, all nations! Extol him, all peoples!”
As the sovereign Son of God and Lord of the church, Jesus simply took up this divine purpose and stated as an absolute certainty: “This gospel of the kingdom will be preached throughout the whole world as a testimony to all the nations.”
The cause of world missions is absolutely assured of success. It cannot fail. Is it not reasonable, then, that we pray with great faith, that we invest with great confidence, and that we go with a sense of sure triumph? (From John Piper, "Then the End Will Come")